A. Field of the Invention
The present invention relates generally to real time communication systems, and more particularly to a method and apparatus for Real Time Protocol feedback mixer traversal.
B. Description of Related Art
Real Time Protocol (RTP) is one type of protocol scheme that can generally provide an end-to-end network transport function. These functions are generally suitable for applications transmitting real-time data. Real time data could include audio, video, simulation data, or a mixture of these types of data. RTP transmits such data over multicast or unicast network services. Network services include payload type identification, sequence numbering, time-stamping, and delivery monitoring. RTP may be used to send real time data across packet switched networks such as a LAN and an Internet.
The Request For Comment 1889 (i.e., RFC 1889), herein entirely incorporated by reference and to which the reader is directed for further details, provides a description of a number of the operating characteristics. According to RFC 1889, RTP consists of two linked parts: Real Time Transport Protocol (RTP) and Real Time Control Protocol (RTCP).
The RTP carries data that has real-time properties. The RTCP monitors the quality of service (QoS) of the real time data transmitted to each network receiver. The RTCP also conveys information covering the various participants involved in a communication session. By the term session, it is meant to include two or more communication nodes involved in or participating in a communications conference such as teleconferencing or video conferencing.
RTCP allows monitoring of transmitted data in a manner that can be scalable to large multicast networks. In other words, monitoring of data transmitted to a large number of communications participants of a real time communications session. RTCP can also provide certain data control and functional identification. RTP and RTCP are designed to be independent of an underlying transport or network layer.
The RTP protocol, as outlined in RFC 1889, supports the use of RTP level translators and mixers. A translator is an intermediate system that forwards RTP packets with their synchronization source (SSRC) identifier intact. Examples of translators include devices that convert encodings without mixing, replicators from unicast to multicast, and application level filters in firewalls.
Mixers may be used where some conference participants in one area are connected through a low-speed link to participants enjoying a higher speed network access. Instead of forcing all conference participants to use a lower-bandwidth, reduced quality audio encoding an RTP-level relay such as a mixer may be used to support the low-bandwidth participants. This mixer can then synchronize encoded audio packets, mix reconstructed audio streams into a single stream, translate the audio encoding to a lower-bandwidth, and forward the lower-bandwidth packet stream across a low speed link.
RTP has a feedback protocol called the Real Time Control Protocol (i.e., RTCP). The RFC 1889 requires a feedback mechanism that allows a source and/or receiver of RTP packet information, such as a communications node, to provide feedback. Such nodal feedback informs a transmitter of information about data transmission characteristics such as quality of service. Feedback characteristics can include information such as percent of packet loss experienced during data stream transmission, a cumulative number of packets lost during a communications session, and information on the packet inter-arrival jitter. RFC 1889, however, requires that the transmitted nodal feedback be transmitted to the mixer. The mixer does not forward this feedback to the originally transmitting node. Consequently, the nodal feedback does not traverse or pass through the mixer. As a result nodes communicating with a mixer have no knowledge of the quality of service experienced at all other nodes communicating with the same mixer. If a message can be developed to allow feedback to traverse a mixer, significant advantages will be realized. Providing feedback information allows conference call participants to alter the digital encoding schemes such that the overall audio and/or video quality may be increased.